Hot formed steel sheet component and method for producing the same as well as steel sheet for hot forming

ABSTRACT

A hot formed steel sheet component including: a chemical composition including in terms of mass % C at from 0.100% to 0.340%, Si at from 0.50% to 2.00%, Mn at from 1.00% to 3.00%, P at 0.050% or less, S at 0.0100% or less, sol. Al at from 0.001% to 1.000%, and N at 0.0100% or less, with a remainder consisting of Fe and impurities; and a steel structure including ferrite, at least one of tempered martensite or tempered bainite, and martensite, wherein an area rate of ferrite is from 5% to 50%, a total area rate of tempered martensite and tempered bainite is from 20% to 70%, an area rate of martensite is from 25% to 75%, a total area rate of ferrite, tempered martensite, tempered bainite and martensite is 90% or more, and an area rate of retained austenite is from 0% to 5%.

TECHNICAL FIELD

The present invention relates to a hot formed steel sheet component to be used, for example, in a machine structural component such as an automobile body structural component, and to a method for producing the same as well as to a steel sheet for hot forming. Specifically, the invention relates to a hot formed steel sheet component having superior ductility and bendability together with a high tensile strength, and to a method for producing the same as well as to a steel sheet for hot forming for yielding the same.

BACKGROUND ART

In recent years, for the sake of weight reduction of an automobile, it has been strived to enhance the strength of a steel to be used for a body so as to reduce an employed weight. In the case of a thin steel sheet used broadly for an automobile, when the strength of a steel sheet is increased, the press formability is compromised so that production of a component with a complicated shape becomes difficult. Specifically, there occurs a problem, for example, that the ductility is lowered so as to cause a fracture at a highly processed region, or that the springback or wall warpage becomes significant so as to impair dimensional accuracy. Therefore, it is not easy to produce such a component by press forming using a steel sheet with a high strength, especially with a tensile strength in a range of 980 MPa or more. Although a high strength steel sheet can be processed not by press forming but by roll forming, it can be applicable only to a component having a uniform cross-section in the longitudinal direction.

On the other hand, by a method called as hot pressing, by which a heated steel sheet is press-formed as disclosed in Patent Document 1, a component having a complicated shape can be formed with high dimensional accuracy, because a steel sheet is soft and highly-ductile at a high temperature. Further, when a steel sheet is heated up to an austenite single phase region and then quenched (hardened) in a mold, enhancement of the strength of a component can be achieved at the same time due to a martensitic transformation. Therefore, such a hot pressing method is a superior forming method, which can attain a high strength of a component and superior formability of a steel sheet simultaneously.

Further, Patent Document 2 discloses a pre-press quenching method, by which a steel sheet is formed in advance to a predetermined shape at room temperature, heated up to an austenite region, and then quenched in a mold to achieve a higher strength of a component. Since such a pre-press quenching method, which is an embodiment of hot pressing, can suppress deformation to be caused by a thermal strain by restraining the component with a mold, it is a superior forming method that secures a higher strength of a component and high dimensional accuracy at the same time.

However, ductility has come to be also required recently for a hot steel sheet component, and conventional art as represented by Patent Document 1 or Patent Document 2, in which the steel structure is substantially a martensite single phase, confronts a problem that such a requirement cannot be satisfied.

With such a background, a hot pressed steel sheet component that is allegedly superior in terms of high strength and ductility owing to the two phase structure of ferrite and martensite, for which a steel sheet is heated to a two phase temperature range of ferrite and austenite, pressed while keeping the two phase structure, and quenched in a mold, is disclosed in Patent Document 3. However, under such a two phase heating condition, a steel structure is apt to become nonuniform, and the bendability and the toughness of a hot pressed steel sheet component may be deteriorated and the impact absorption characteristic may be impaired extremely.

Meanwhile, Patent Document 4 discloses a hot pressed steel sheet component, which is yielded by heating a steel sheet having a steel structure with 80 volume-% or more of martensite or bainite at an Ac₁ transformation point or higher, and then quenching it in a mold to have a structure containing from 3 to 20 volume-% of retained austenite, from 30 to 97 volume-% of tempered martensite or tempered bainite, and from 0 to 67 volume-% of martensite, and is allegedly superior in terms of high strength and ductility.

Furthermore, Patent Document 5 discloses a high strength pressed component satisfying that the area rate of martensite with respect to the entire steel sheet structure is from 10% to 85%, 25% or more of the martensite is tempered martensite, the content of retained austenite is from 5% to 40%, the area rate of bainitic ferrite in bainite with respect to the entire steel sheet structure is 5% or more, and a total of the area rate of martensite, the area rate of retained austenite, and the area rate of bainitic ferrite in the bainite with respect to the entire steel sheet structure is 65% or more.

Patent Document 6 discloses a steel sheet for hot pressing in which a total fraction of bainite and martensite is 80% by area or more.

Further, Patent Document 7 discloses a steel sheet for hot pressing in which the fraction of ferrite is 30% by area or more.

CITATION LIST Patent Literature

-   Patent Document 1: GB Patent No. 1490535 -   Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No.     H10-96031 -   Patent Document 3: JP-A No. 2010-65292 -   Patent Document 4: JP-A No. 2012-237066 -   Patent Document 5: International Publication No. WO2011/111333 -   Patent Document 6: JP-A No. 2013-185243 -   Patent Document 7: JP-A No. 2013-185248

SUMMARY OF INVENTION Technical Problem

It has become clear through investigation by the present inventors that by making the steel structure of a steel sheet for hot pressing contain mainly bainite or martensite, not only the ductility of a hot pressed steel sheet component as described, for example, in Patent Literature 4, but also the toughness is improved. However, even by such regulation of the structure of a component, deterioration of the bendability cannot be resolved, and a bending crack in a component to appear at a buckling region during impact deformation cannot be prevented. This drawback becomes obvious when the tensile strength of a steel becomes high (for example, 980 MPa or more). With respect to a hot pressed steel sheet component with a high tensile strength (for example, tensile strength of 980 MPa or more) and superior in bendability in addition to ductility, a product itself has been heretofore not yet proposed, let alone establishment of a production technology.

Similarly, even when the whole hot formed steel sheet components including a roll-formed component besides a hot pressed steel sheet component are surveyed, with respect to a hot formed steel sheet component with a high tensile strength (for example, tensile strength of 980 MPa or more) and superior in bendability in addition to ductility, a product itself has been heretofore not yet proposed, let alone establishment of a production technology.

A specific object of the invention is to provide a hot pressed steel sheet component, which has a high tensile strength and is superior in ductility and bendability after hot pressing, not available according to conventional art as described above, and a method for producing the same as well as a steel sheet for hot pressing for yielding the same. By generalization, the invention is also applicable to hot forming provided with a means for cooling a steel sheet simultaneously during or immediately after forming as in the case of hot pressing. Therefore, a specific object of the invention is to provide a hot formed steel sheet component superior in ductility and bendability while having a high tensile strength after hot forming, and a method for producing the same as well as a steel sheet for hot forming for yielding the same.

Solution to Problem

The inventors studied diligently for improving the ductility and the bendability of a hot formed steel sheet component with a high tensile strength. As the result, the following novel knowledge has been acquired. Namely, a steel sheet for hot forming including a chemical composition in which Si is actively added to specific amounts of C and Mn, and also including a steel structure containing ferrite and at least one of martensite or bainite is to be utilized. Further, a heat treatment condition for hot forming optimum to the steel sheet for hot forming is to be applied. By the above means, differently from a conventional hot formed steel sheet component, the steel structure can be made to include a dual phase, which contains no, or not more than 5% of, retained austenite in terms of area rate, and contains ferrite, at least one of tempered martensite or tempered bainite, and martensite at predetermined area rates. As a result, a novel knowledge has been acquired that a hot formed steel sheet component superior in ductility and bendability while having a high tensile strength can be produced, when the chemical composition and the steel structure are present.

The invention based on the knowledges is as follows:

[1] A hot formed steel sheet component comprising:

a chemical composition comprising in terms of mass % C at from 0.100% to 0.340%, Si at from 0.50% to 2.00%, Mn at from 1.00% to 3.00%, P at 0.050% or less, S at 0.0100% or less, sol. Al at from 0.001% to 1.000%, and N at 0.0100% or less, with a remainder consisting of Fe and impurities; and

a steel structure comprising ferrite, at least one of tempered martensite or tempered bainite, and martensite, wherein an area rate of ferrite is from 5% to 50%, a total area rate of tempered martensite and tempered bainite is from 20% to 70%, an area rate of martensite is from 25% to 75%, a total area rate of ferrite, tempered martensite, tempered bainite and martensite is 90% or more, and an area rate of retained austenite is from 0% to 5%.

[2] The hot formed steel sheet component according to Paragraph [1], wherein the chemical composition comprises one kind or two or more kinds selected from the group consisting of Ti at 0.200% or less, Nb at 0.200% or less, V at 0.200% or less, Cr at 1.000% or less, Mo at 1.000% or less, Cu at 1.000% or less, and Ni at 1.000% or less, in terms of mass % instead of a part of the Fe. [3] The hot formed steel sheet component according to Paragraph [1] or Paragraph [2], wherein the chemical composition comprises B at 0.0025% or less, in terms of mass % instead of a part of the Fe. [4] The hot formed steel sheet component according to any one of Paragraph [1] to Paragraph [3], wherein the chemical composition comprises one kind or two or more kinds selected from the group consisting of Ca at 0.0100% or less, Mg at 0.0100% or less, REM at 0.0100% or less, and Zr at 0.0100% or less, in terms of mass % instead of a part of the Fe. [5] The hot formed steel sheet component according to any one of Paragraph [1] to Paragraph [4], wherein the chemical composition comprises Bi at 0.0100% or less, in terms of mass % instead of a part of the Fe. [6] A steel sheet for hot forming comprising:

a chemical composition comprising in terms of mass % C at from 0.100% to 0.340%, Si at from 0.50% to 2.00%, Mn at from 1.00% to 3.00%, P at 0.050% or less, S at 0.0100% or less, sol. Al at from 0.001% to 1.000%, and N at 0.0100% or less, with a remainder consisting of Fe and impurities; and

a steel structure comprising ferrite with an aspect ratio of 2.0 or less, and at least one of martensite or bainite, wherein an area rate of ferrite is from 5% to 50%, a total area rate of martensite and bainite is from 45% to 90%, and a total area rate of ferrite, martensite, and bainite is 90% or more.

[7] The steel sheet for hot forming according to Paragraph [6], wherein the chemical composition comprises one kind or two or more kinds selected from the group consisting of Ti at 0.200% or less, Nb at 0.200% or less, V at 0.200% or less, Cr at 1.000% or less, Mo at 1.000% or less, Cu at 1.000% or less, and Ni at 1.000% or less, in terms of mass % instead of a part of the Fe. [8] The steel sheet for hot forming according to Paragraph [6] or Paragraph [7], wherein the chemical composition comprises B at 0.0025% or less, in terms of mass % instead of a part of the Fe. [9] The steel sheet for hot forming according to any one of Paragraph [6] to Paragraph [8], wherein the chemical composition comprises one kind or two or more kinds selected from the group consisting of Ca at 0.0100% or less, Mg at 0.0100% or less, REM at 0.0100% or less, and Zr at 0.0100% or less, in terms of mass % instead of a part of the Fe. [10] The steel sheet for hot forming according to any one of Paragraph [6] to Paragraph [9], wherein the chemical composition comprises Bi at 0.0100% or less, in terms of mass % instead of a part of the Fe. [11] A method for producing a hot formed steel sheet component, the method comprising: heating the steel sheet for hot forming according to any one of Paragraph [6] to Paragraph [10] to a temperature range of 720° C. or higher but lower than an Ac₃ point; performing hot forming within a time period of from 3 sec to 20 sec, during which the steel sheet is exposed to air cooling from the end of the heating until the initiation of the hot forming; and cooling to a temperature range not above an M_(S) point at an average cooling rate of from 10° C./sec to 500° C./sec.

Advantageous Effects of Invention

A technologically valuable effect that a hot formed steel sheet component, which, as hot formed, has a high tensile strength, and is superior in ductility a well as bendability, can be at last put into practical use, has been attained by the invention. A hot formed steel sheet component according to the invention exhibits extremely superior collision characteristics, such that it can absorb an impact through a bending deformation even at a collision causing most severe plastic deformation. Therefore, a hot formed steel sheet component according to the invention is especially suitable for producing a structural component of an automobile body, however it is naturally applicable to another use such as a machine structural component.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a photograph showing an example of a steel structure according to the invention.

DESCRIPTION OF EMBODIMENTS

Next, reasons behind limitations of each range imposed according to the invention will be described. In this regard, hot forming will be described below taking hot pressing, which is a specific embodiment thereof, as an example. A numerical range expressed by “x to y” includes the values of x and y in the range as the minimum and maximum values, respectively.

1. Chemical Composition

Firstly, reasons behind the above definition of the chemical composition of a hot formed steel sheet component according to the invention (hereinafter also referred to simply as “steel sheet component”) and a steel sheet for hot forming according to the invention (hereinafter also referred to simply as “steel sheet”) will be described. In the following descriptions, “%” representing the content of each alloy element means “mass %” unless otherwise specified.

(C at 0.100% to 0.340%)

C is a very important element, which enhances the hardenability of a steel and predominantly decides the strength after hot pressing (after quenching). When the C content is less than 0.100%, it becomes difficult to secure a high tensile strength (for example, tensile strength of 980 MPa or more) after hot pressing (after quenching). Therefore the C content should be 0.100% or more, and is preferably 0.120% or more. Meanwhile, when the C content is above 0.340%, martensite after hot pressing (after quenching) becomes rigid, such that not only deterioration of the bendability becomes remarkable, but also the ductility declines. Therefore, the C content should be 0.340% or less. From the viewpoint of weldability, the C content is preferably 0.300% or less and still more preferably 0.280% or less.

(Si at 0.50% to 2.00%)

Si is a very effective element for enhancing the ductility of a steel heated into a two phase temperature range of ferrite and austenite, and securing a stable strength after hot pressing (after quenching). When the Si content is less than 0.50%, the effects are difficult to obtain. Therefore, the Si content should be 0.50% or more. From the viewpoint of improvement of weldability, the Si content is preferably 0.70% or more, and still more preferably 1.10% or more. Meanwhile, when the Si content is above 2.00%, the effect of the above action becomes saturated, which is economically disadvantageous, and defective plating occurs frequently due to a remarkable decrease in plating wettability. Therefore, the Si content should be 2.00% or less. From the viewpoint of suppression of surface defects in a hot formed steel sheet component, the Si content is preferably 1.80% or less, and still more preferably 1.50% or less.

(Mn at 1.00% to 3.00%)

Mn is a very effective element for improving the hardenability of a steel and securing a strength after hot pressing (after quenching). However, when the Mn content is less than 1.00%, not only it becomes very difficult to secure a high tensile strength (for example, tensile strength of 980 MPa or more) after hot pressing (after quenching), but also the bendability may be compromised. Therefore, the Mn content should be 1.00% or more. For securing the action more stably, the Mn content is preferably 1.10% or more, and still more preferably 1.20% or more. Meanwhile, when the Mn content is above 3.00%, a steel structure after hot pressing (after quenching) shows an obvious band caused by Mn segregation, which compromises the toughness and remarkably deteriorates the collision characteristics. Therefore, the Mn content should be 3.00% or less. From the viewpoint of the productivity of hot rolling and cold rolling, the Mn content is preferably 2.50% or less and still more preferably 2.40% or less.

By defining C, Si and Mn within the above ranges, the steel structure of a steel sheet for hot forming can be made to a steel structure with a dual phase containing ferrite and at least one of martensite or bainite. Further, by defining the heating conditions during hot pressing according to the invention, the steel structure of a hot formed steel sheet component can be made to a desired steel structure with a dual phase.

(P at 0.050% or Less)

Although P is generally an impurity contained in a steel, since it has an action to enhance the strength of a steel sheet by solid solution strengthening, it may be added actively. However, when the P content is above 0.050%, deterioration of the weldability becomes conspicuous. Therefore, the P content should be 0.050% or less. The P content is preferably 0.018% or less. For securing an effect of the action, the P content is preferably 0.003% or more.

(S at 0.0100% or Less)

S is an impurity contained in a steel, and the content is preferably as low as possible from a viewpoint of weldability. When the S content is above 0.0100%, deterioration of the weldability becomes conspicuous. Therefore, the S content should be 0.0100% or less. The S content is preferably 0.0030% or less, and still more preferably 0.0015% or less. From the viewpoint of desulfurization costs, the S content is preferably 0.0006% or more.

(Sol. Al (Soluble Al) at 0.001% to 1.000%)

Al is an element having an action to make the quality of a steel robust through deoxidation. When the sol. Al content is less than 0.001%, it becomes difficult to obtain the action. Therefore, the sol. Al content should be 0.001% or more, and is preferably 0.015% or more. Meanwhile, when the sol. Al content is above 1.000%, deterioration of the weldability becomes conspicuous, and further an oxide-type inclusion increases so that deterioration of a surface condition becomes also conspicuous. Therefore, the sol. Al content should be 1.000% or less, and is preferably 0.080% or less. In this regard, sol. Al means acid-soluble Al, which is not in a form of an oxide such as Al₂O₃, and soluble in an acid.

(N at 0.0100% or Less)

N is an impurity contained in a steel, and the content is preferably as low as possible from a viewpoint of weldability. When the N content is above 0.0100%, deterioration of the weldability becomes conspicuous. Therefore, the N content should be 0.0100% or less, and is preferably 0.0060% or less. From a viewpoint of denitrification cost, the N content is preferably 0.0020% or more.

[Impurities]

Impurities refer to ingredients contained in a source material or ingredients entered during a process of production, which are ingredients that are not intentionally added to a steel sheet component or a steel sheet for hot forming.

The chemical compositions of a steel sheet component and a steel sheet for hot forming according to the invention may contain further at least one kind of the elements described below.

(One Kind or Two or More Kinds Selected from the Group Consisting of Ti at 0.200% or Less, Nb at 0.200% or Less, V at 0.200% or Less, Cr at 1.000% or Less, Mo at 1.000% or Less, Cu at 1.000% or Less, and Ni at 1.000% or Less)

Each of the elements is an element having an effect of securing a stably high strength after hot pressing (after quenching). Therefore, one kind or two or more kinds of the elements may be added. However, with respect to Ti, Nb and V, when any of them is contained beyond 0.200%, not only hot rolling and cold rolling may become difficult, but also securement of a stably high strength may become difficult. Therefore, preferably the Ti content, the Nb content, and the V content are respectively 0.200% or less. With respect to Cr, when the content exceeds 1.000%, securement of a stably high strength may become difficult. Therefore, the Cr content is preferably 1.000% or less. With respect to Mo, when the content exceeds 1.000%, hot rolling and cold rolling may become difficult. Therefore, the Mo content is preferably 1.000% or less. With respect to Cu and Ni, when the content of any of them exceeds 1.000%, an effect of the action is apt to be saturated so that the economy may become disadvantageous, and further hot rolling or cold rolling may become difficult. Therefore, preferably the Cu content, and the Ni content are respectively 1.000% or less.

For securing an effect of the action, it is preferable to satisfy at least one of Ti at 0.003% or more, Nb at 0.003% or more, V at 0.003% or more, Cr at 0.005% or more, Mo at 0.005% or more, Cu at 0.005% or more, or Ni at 0.005% or more.

In other words, the lower limit of the Ti content is preferably 0.003%. The lower limit of the Nb content is preferably 0.003%. The lower limit of the V content is preferably 0.003%. The lower limit of the Cr content is preferably 0.005%. The lower limit of the Mo content is preferably 0.005%. The lower limit of the Cu content is preferably 0.005%. The lower limit of the Ni content is preferably 0.005%.

(B at 0.0025% or Less)

B is an element having an action to enhance the toughness of a steel. Therefore, B may be added. However, when B is added in an amount exceeding 0.0025%, it may become difficult for the steel structure in a steel sheet for hot forming to contain ferrite, and the ductility and the bendability of a hot formed steel sheet component may be deteriorated. Therefore, the B content should be preferably 0.0025% or less. Further, for securing an effect of the action, the B content is preferably 0.0003% or more.

(One Kind or Two or More Kinds Selected from the Group Consisting of Ca at 0.0100% or Less, Mg at 0.0100% or Less, REM at 0.0100% or Less, and Zr at 0.0100% or Less)

Any of these elements is an element having an action to enhance the toughness through contribution to control of inclusion, especially to microdispersion of inclusions. Therefore, one kind or two or more kinds of the elements may be added. However, when the content of any element exceeds 0.0100%, deterioration of a surface condition may become conspicuous. Therefore, the content of each element is preferably 0.0100% or less. For obtaining an effect of the action more securely, the content of at least one of the elements should be preferably 0.0003% or more. Namely, preferably the lower limits of the Ca content, the Mg content, the REM content, and the Zr content are respectively 0.0003%.

In this regard, REM represents at least one kind of a total of 17 elements including Sc, Y, and lanthanoids. The above REM content means a total content of at least one kind of the elements. A lanthanoid is industrially added in a form of a misch metal.

(Bi at 0.0100% or Less)

Bi is an element having an action to make the structure uniform and enhance the bendability. Therefore, Bi may be contained. However, when Bi is added more than 0.0100%, the hot working property may be deteriorated so that hot rolling may become difficult. Therefore, the Bi content should be preferably 0.0100% or less. For obtaining an effect of the action more securely, the Bi content should be preferably 0.0003% or more.

2. Steel Structure of Hot Formed Steel Sheet Component

Next, the steel structure of a hot formed steel sheet component according to the invention will be described.

A hot formed steel sheet component according to the invention includes a steel structure containing ferrite, at least one of tempered martensite or tempered bainite, and martensite at the following predetermined area rates. In other words, the steel structure may contain one of tempered martensite or tempered bainite, or contain both of the same. Further, the steel structure contains no retained austenite, or contains the same only not more than 5% by area rate.

FIG. 1 shows an example of a steel structure according to the invention. The steel structure shown in FIG. 1 is a steel structure containing ferrite, tempered martensite, and martensite, but not containing retained austenite.

(Area Rate of Ferrite at 5% to 50%)

When the area rate of ferrite is less than 5%, the ductility and the bendability decline. Therefore, the area rate of ferrite should be 5% or more, and is preferably 15% or more. Meanwhile, when the area rate of ferrite is above 50%, the bendability declines. Therefore, the area rate of ferrite should be 50% or less, and is preferably 40% or less,

The aspect ratio of ferrite is preferably 2.0 or less from a viewpoint of suppression of decline in bendability. When the aspect ratio of ferrite exceeds 2.0, the anisotropy of ferrite (crystal grain of ferrite) increases, and such ferrite may constitute an origin of stress concentration, and the bendability may decline. Therefore, the aspect ratio of ferrite is preferably 2.0 or less, and more preferably 1.8 or less. Meanwhile, when the aspect ratio of ferrite approaches closer to 1.0, the anisotropy of ferrite (crystal grain of ferrite) decreases further, therefore the lower limit of the aspect ratio of ferrite is preferably 1.0. However, from a viewpoint of enhancement of the yield strength of a steel sheet component after hot pressing, the lower limit of the aspect ratio of ferrite is preferably 1.2.

An aspect ratio of ferrite is a value measured by the method described precisely in Example presented below.

(Total Area Rate of Tempered Martensite and Tempered Bainite at 20% to 70%)

When the total area rate of tempered martensite and tempered bainite is less than 20%, the bendability declines. Therefore, the total area rate of tempered martensite and tempered bainite should be 20% or more, and is preferably 30% or more. Meanwhile, when the total area rate of tempered martensite and tempered martensite is above 70%, the ductility declines. Therefore, the total area rate of tempered martensite and tempered bainite should be 70% or less, and is preferably 50% or less.

(Area Rate of Martensite at 25% to 75%)

By forming martensite in a steel, the strength after hot pressing (after quenching) can be enhanced. When the area rate of martensite is less than 25%, it becomes difficult to secure a high tensile strength (for example, tensile strength of 980 MPa or more) after hot pressing (after quenching). Therefore, the area rate of martensite should be 25% or more. Meanwhile, when the area rate of martensite is above 75%, the ductility declines. Therefore, the area rate of martensite should be 75% or less, and is preferably 50% or less.

In this regard, “martensite” means both of as-quenched martensite, and martensite after age hardening formed by age-hardening as-quenched martensite. Namely, the “area rate of martensite” means the total area rate of as-quenched martensite, and martensite after age hardening formed by age-hardening as-quenched martensite.

(Total Area Rate of Ferrite, Tempered Martensite, Tempered Bainite, and Martensite at 90% or More)

Basically, a hot formed steel sheet component according to the invention has a structure containing ferrite, tempered martensite, tempered bainite, and martensite. However, depending on a production condition, as a phase or a structure other than the above, one kind or two or more kinds of bainite, retained austenite, cementite, or pearlite may be mixed in. In this case, when the percentage of such a phase or a structure other than ferrite, tempered martensite, tempered bainite, and martensite exceeds 10%, an intended characteristic may not be obtained due to an influence of the phase or structure. Therefore, the mixture of a phase or a structure other than ferrite, tempered martensite, tempered bainite, and martensite should be 10% or less, and is preferably 5% or less. Namely, the total area rate of ferrite, tempered martensite, tempered bainite, and martensite should be 90% or more, and preferably 95% or more. The upper limit of the total area rate of ferrite, tempered martensite, tempered bainite, and martensite is 100%.

(Area Rate of Retained Austenite at 0% to 5%)

With respect to a phase or a structure other than ferrite, tempered martensite, tempered bainite, and martensite, when especially retained austenite is mixed (retained) more than 5% in terms of area rate, the bendability declines. Therefore, retained austenite should not be contained, or even if it should be contained, the area rate of retained austenite should be 5% or less, and is preferably 3% or less. The area rate of retained austenite is most preferably 0%.

The area rate of each phase and structure in the steel structure of a hot formed steel sheet component is a value measured by the method described precisely in Example presented below.

A steel sheet component according to the invention means a component hot formed from a steel sheet, and includes, for example, a steel sheet component formed by hot pressing. As a typical example, there is a door guard bar to be used as an automobile body structural component. Further, for an automobile use, there is, for example, a bumper reinforcement. As a machine structural component, there is a hot formed steel pipe for a building structure produced from a steel sheet as a source material.

3. Mechanical Properties

It is preferable that a hot formed steel sheet component according to the invention has a tensile strength (TS) of 980 MPa or more, which is adequate to contribute to weight reduction of an automobile.

4. Production Method

Next, a preferable method for producing a hot formed steel sheet component according to the invention having the above characteristics will be described.

In order to attain favorable ductility and bendability for a hot formed steel sheet component according to the invention, while securing a high tensile strength (for example, tensile strength of 980 MPa or more), the steel structure after hot pressing (after quenching) should better, as described above, not be a martensite single phase, but rather be a dual phase structure, in which the area rate of ferrite is from 5% to 50%, the total area rate of tempered martensite, and tempered bainite is from 20% to 70%, the area rate of martensite is from 25% to 75%, the total area rate of ferrite, tempered martensite, tempered bainite, and martensite is 90% or more, as well as the area rate of retained austenite is from 0% to 5%.

In order to obtain a steel structure for a hot formed steel sheet component according to the invention, as a steel sheet (steel sheet for hot forming), which is a source material for hot forming, a steel sheet including the above chemical composition, and a steel structure (dual phase structure) containing ferrite with an aspect ratio of 2.0 or less, and at least one of martensite or bainite, in which structure the area rate of ferrite is from 5% to 50%, the total area rate of martensite and bainite is from 45% to 90%, the total area rate of ferrite, martensite, and bainite is 90% or more, is preferably used. Then, preferably the steel sheet (steel sheet for hot forming) is heated into a temperature range of 720° C. or higher but lower than an Ac₃ point, then performed to hot pressing within a time period of from 3 sec to 20 sec, during which the steel sheet is exposed to air cooling from the end of the heating until the initiation of the hot pressing, and then cooled to a temperature range not above an M_(S) point at an average cooling rate of from 10° C./sec to 500° C./sec.

By hot pressing a steel sheet for hot forming including the chemical composition and the steel structure under the above conditions, a hot formed steel sheet component having a desired steel structure after hot pressing, with a high tensile strength (for example, tensile strength of 980 MPa or more), and superior in ductility and bendability can be obtained.

(Steel Structure of Steel Sheet for Hot Forming)

—Aspect Ratio of Ferrite at 2.0 or Less—

When the aspect ratio of ferrite is above 2.0, the aspect ratio of ferrite in a steel structure of a steel sheet component after hot pressing may also exceed 2.0, and further the ferrite area rate in a steel sheet component after hot pressing may fall below 5%, because ferrite is transformed excessively to austenite during heating. When the aspect ratio of ferrite of steel sheet component exceeds 2.0, the anisotropy of ferrite (crystal grain of ferrite) increases and constitutes an origin of stress concentration, so that the bendability may decline. Therefore, the aspect ratio of ferrite should be 2.0 or less, and is preferably 1.8 or less. Meanwhile, when the aspect ratio of ferrite approaches closer to 1.0, the anisotropy of ferrite (crystal grain of ferrite) decreases further, therefore the lower limit of the aspect ratio of ferrite is preferably 1.0. However, from a viewpoint of enhancement of the yield strength of a steel sheet component after hot pressing, the lower limit of the aspect ratio of ferrite is preferably 1.2.

An aspect ratio of ferrite is a value measured by the method described precisely in Example presented below.

—Area Rate of Ferrite at 5% to 50% —

When the area rate of ferrite is less than 5%, the area rate of ferrite in a steel structure of a steel sheet component after hot pressing may also become less than 5%. Therefore, the area rate of ferrite should be 5% or more, and is preferably 15% or more. Similarly, when the area rate of ferrite is above 50%, the area rate of ferrite in a steel structure of a steel sheet component after hot pressing may also exceed 50%. Therefore, the area rate of ferrite should be 50% or less, and is preferably 45% or less.

—Total Area Rate of Martensite and Bainite at 45% to 90%—

When the total area rate of martensite and bainite is less than 45%, the total area rate of tempered martensite and tempered bainite in the steel structure of a steel sheet component after hot pressing may become less than 20%. Further, the area rate of martensite in the steel structure of a steel sheet component after hot pressing may become less than 25%. Therefore, the total area rate of martensite and bainite should be 45% or more, and is preferably 50% or more. Similarly, when the total area rate of martensite and bainite is above 90%, the total area rate of tempered martensite and tempered bainite in the steel structure of a steel sheet component after hot pressing may also exceed 70%. Further, the area rate of martensite in the steel structure of a steel sheet component after hot pressing may exceed 75%. Therefore, the total area rate of martensite and bainite should be 90% or less, and is preferably 80% or less.

—Total Area Rate of Ferrite, Martensite, and Bainite at 90% or More—

When the total area rate of ferrite, martensite, and bainite is less than 90%, mixture of a phase or a structure other than ferrite, tempered martensite, tempered bainite, and martensite in the steel structure of a steel sheet component after hot pressing may exceed 10%. Especially, the area rate of retained austenite may exceed 5%. Therefore, the total area rate of ferrite, martensite, and bainite should be 90% or more, and is preferably 93% or more. The upper limit of the total area rate of ferrite, martensite, and bainite is 100%.

An area rate of each phase and structure in the steel structure of a steel sheet for hot forming is a value measured by the method described precisely in Example presented below.

(Production of Steel Sheet for Hot Forming)

A steel sheet for hot forming may be any of a hot-rolled steel sheet, a cold-rolled steel sheet, and a coated steel sheet. Examples of a coated steel sheet include aluminum coated steel sheet, and zinc coated steel sheet.

A hot-rolled steel sheet having the above steel structure can be produced in a hot rolling step, by defining C, Si and Mn within the ranges in terms of the chemical composition, and completing finish-rolling at from 850° C. to 930° C., retaining the product in process in a range from 740° C. to 660° C. for 3 sec or more, and winding the same in a temperature range of 450° C. or less. Further, a cold-rolled steel sheet having the above steel structure can be produced, after cold rolling, by heating the product in process at from 780° C. to 900° C., and then cooling the same at an average cooling rate of 10° C./sec or more in an annealing step. A coated steel sheet having the above steel structure can be produced, after production of the hot-rolled steel sheet or the cold-rolled steel sheet, by performing a well known plating treatment on a surface of the hot-rolled steel sheet or the cold-rolled steel sheet.

(Heating of Steel Sheet for Hot Forming: Heating to Temperature Range of 720° C. or Higher but Lower than an Ac₃ Point)

Heating of a steel sheet for hot forming is performed up to a temperature of 720° C. or higher but lower than an Ac₃ point. In this regard, the Ac₃ point (° C.) is a temperature defined by the following empirical Formula (i), which is lower than the Ac₃ point (° C.) of an austenite single phase.

Ac₃=910-203×(C^(0.5))−15.2×Ni+44.7×Si+104×V+31.5×Mo−30×Mn−11×Cr−20×Cu+700×P+400×sol. Al+50×Ti  (i)

In this case the symbols of elements in Formula (i) represent the contents of the respective elements (by mass %) in the chemical composition of a steel sheet. In this regard, Formula (i) is calculated by putting the content of an element not contained in a steel sheet as 0 (0 mass %).

When a heating temperature is less than 720° C., austenitization becomes insufficient and martensite is not contained in a hot pressed steel sheet, so that securement of a high tensile strength (for example, tensile strength of 980 MPa or more) after hot pressing (after quenching) becomes difficult. Therefore, a heating temperature should be 720° C. or higher, and is preferably 750° C. or higher. Meanwhile, when a heating temperature is not lower than an Ac₃ point, even if a steel sheet is exposed to air cooling thereafter, the area rate of martensite in a steel structure after hot pressing (after quenching) exceeds 75%, and deterioration of the ductility becomes remarkable. Therefore, a heating temperature should be not higher than an Ac₃ point, and is preferably not higher than an Ac₃ point −30° C.

Although there is no particular restriction on a heating rate up to 720° C. and a heating time for retention in the temperature range, they are preferably in the following ranges respectively.

The average heating rate in heating up to 720° C. is preferably from 0.2° C./sec to 100° C./sec. When the average heating rate is 0.2° C./sec or more, high productivity can be secured. Further, when the average heating rate is 100° C./sec or less, the heating temperature can be regulated easily, even in a case in which heating is conducted in an ordinary furnace.

The heating time in a temperature range of 720° C. or higher and lower than an Ac₃ point is preferably from 2 min to 10 min. In this regard, a heating time is a time period from a time point, when the temperature of a steel sheet reaches 720° C., to a time point of completion of heating. Specifically, the time point of completion of heating means, in the case of furnace heating, a time point, when a steel sheet is taken out of a heating furnace, and in the case of Joule heating or induction heating, a time point, when the power supply is cut off. When the heating time is 2 min or more, the strength after hot pressing (after quenching) can be made more stable. When the retention time is 10 min or less, the structure of a steel sheet component can be micronized further, so that the toughness of a steel sheet component can be further improved.

(Time Period from Completion of Heating to Initiation of Hot Pressing, During which Steel Sheet being Exposed to Air Cooling: 3 Sec to 20 Sec)

In general, a steel sheet for hot forming is transported after heating in a heating furnace to a hot press. In this case, for example, during extraction from a heating furnace, or during transportation to or loading on a hot press, a steel sheet may be partly exposed to air cooling. Since ferrite is newly formed or grown during such air cooling, the time duration of air cooling has influence on tensile strength. Therefore, for securing stably a high strength after hot pressing (after quenching), the time duration of air cooling should be preferably short. Especially, when the time period from completion of heating to initiation of hot pressing, during which a steel sheet is exposed to air cooling, is above 20 sec, the tensile strength of a steel sheet component after hot pressing (after quenching) decreases, or even when a high tensile strength (for example, tensile strength of 980 MPa or more) is secured, carbon concentration in austenite becomes conspicuous and martensite transformed region is apt to crack, so that the bendability declines. Therefore, the time period from completion of heating to initiation of hot pressing, during which a steel sheet is exposed to air cooling, should be 20 sec or less, and is preferably 16 sec or less. Meanwhile, austenite formed during heating has precipitated in an acicular form. Since a part of the precipitated austenite transforms during cooling to ferrite and the form of the austenite changes gradually from an acicular form to a spherical form, when hot pressing (quenching) is conducted within a time period of less than 3 sec from completion of heating to initiation of hot pressing, during which a steel sheet is exposed to air cooling, to cause martensitic transformation, an acicular martensitic transformed region constitutes an origin of stress concentration, so that not only the bendability declines, but also retained austenite is apt to be formed. Therefore, the time period from completion of heating to initiation of hot pressing, during which a steel sheet is exposed to air cooling, should be 3 sec or more, and is preferably 7 sec or more, more preferably 10 sec or more.

In this regard, the time period allowing exposure to air cooling can be regulated by regulating a transportation time from extraction out of a heating furnace to a press mold, which is ordinarily exposed to air cooling.

(Average Cooling Rate to Temperature Range of M_(S) Point or Lower: From 10° C./Sec to 500° C./Sec)

When a steel sheet for hot forming is hot-pressed and cooled to a temperature range of an M_(S) point (M_(S) point=starting temperature of martensitic transformation) or lower at an average cooling rate of from 10° C./sec to 500° C./sec, diffusional transformation is suppressed. When the average cooling rate is less than 10° C./sec, bainitic transformation advances excessively. Alternatively, pearlitic transformation occurs, so that the area rate of martensite, which is a reinforcing phase, cannot be secured, and a high tensile strength (for example, tensile strength of 980 MPa or more) after hot pressing (after quenching) is difficult to secure. Possibly, austenite is stabilized, so that the bendability declines. Therefore, the average cooling rate in the temperature range should be 10° C./sec or more, and is preferably 30° C./sec or more. Meanwhile, when the average cooling rate is over 500° C./sec, it becomes extremely difficult to maintain the soaking of a steel sheet component, and the strength becomes unstable. Therefore, the average cooling rate should be 500° C./sec or less, and is preferably 200° C./sec or less.

In this regard, an average cooling rate is a value obtained by dividing the deference between a temperature for performing hot pressing (° C.) and an Ms point (° C.) by a time period required from the temperature for performing hot pressing (° C.) to the Ms point (° C.).

In this regard, during cooling extremely large heat evolution due to phase transformation occurs after reaching 400° C., and therefore adequate cooling rate may not be secured by the same cooling method to be used in a temperature range not lower than 400° C. Therefore, cooling from 400° C. to an M_(S) point is required to be performed more strongly than cooling down to 400° C., and preferably performed as specified below. By a hot pressing method, cooling is achieved ordinarily by a steel-made mold at normal temperature or several tens of degrees Celsius. Therefore, for changing a cooling rate, a mold dimension may be changed so as to change the heat capacity. Further, the cooling rate can be changed also by changing a mold material to a different metal (for example, copper). In a case in which the mold dimension cannot be changed, a water-cooled mold may be used to change the cooling rate by changing the cooling water amount. Further, using a mold, on which several grooves are cut in advance, the cooling rate may be changed by flowing water in the groves during pressing, or the cooling rate may be also changed by lifting a press interrupting pressing and flowing water in between. Furthermore, the cooling rate may be also changed by changing a mold clearance so as to change the contact area with a steel sheet. As a means for changing the cooling rate, for example, above and below 400° C., the following means are conceivable.

(1) Immediately after arriving at 400° C., a component is transferred to a mold having a different heat capacity or a mold at a room temperature condition, to change a cooling rate. (2) In the case of a water-cooled mold, immediately after arriving at 400° C., the water flow rate in the mold is changed to change a cooling rate. (3) Immediately after arriving at 400° C., water is flown between a mold and a component and the water flow rate is changed to change a cooling rate.

There is no particular restriction on a form of forming by a hot pressing method according to the invention. Examples thereof include bending, draw forming, stretch-expand forming, bore expansion forming, and flange forming. Suitable one may be selected appropriately according to a kind of an intended hot formed steel sheet component. Typical examples of a hot formed steel sheet component include a door guard bar and a bumper reinforcement, which are automobile reinforcing components, as described above.

A hot formed steel sheet component according to the invention is characterized in that it is superior in ductility and bendability. As ductility to withstand practical use, total elongation in a tensile test of 12% or more is preferable, and total elongation of 14% or more is still more preferable. As bendability, a limit bending radius of 5 t or less in a V-bend test with a tip angle of 90° is preferable.

A hot formed steel sheet component after hot pressing may be subjected to a shotblasting treatment in order to remove a scale. The shotblasting treatment has an effect of introducing a compression stress in a surface, and therefore offers an advantage of suppressing a delayed fracture and also enhancing the fatigue strength.

Although in the above description, hot forming has been described taking hot pressing, which is its specific embodiment, as an example, the invention is applicable similarly to hot pressing also to hot forming provided with a means for cooling a steel sheet simultaneously with or immediately after forming, for example to roll forming.

EXAMPLES

Examples according to the invention will be described, provided that the invention be in no way restricted by Examples.

Steel sheets having chemical compositions set forth in Table 1 were used as test materials. Each of the steel sheets was prepared by heating a slab ingoted in a laboratory at 1,250° C. for 30 min, then, except test materials No. 6 and No. 22, subjected to hot rolling, such that finish-rolling is completed in a range of from 880° C. to 910° C., and the material is retained in a range of from 720° C. to 680° C. for 5 sec, to yield a 2.6 mm-thick hot-rolled steel sheet. After hot rolling, the sheet was cooled by water spraying down to 420° C. or lower, and then cooled slowly at 20° C./hour to room temperature, simulating a step for winding a hot rolled sheet in a temperature range of 420° C. or lower.

TABLE 1 Chemical composition (unit: mass %, remainder: Fe and impurities) Steel C Si Mn P S sol. Al N Ti Nb V Cr Mo A 0.167 1.20 1.51 0.014 0.0013 0.035 0.0042 0.150 B 0.200 0.25 1.42 0.014 0.0015 0.033 0.0046 0.021 C 0.139 1.46 2.03 0.012 0.0009 0.029 0.0043 0.023 D 0.171 1.17 1.88 0.014 0.0013 0.037 0.0046 0.017 E 0.202 1.23 1.64 0.013 0.0011 0.033 0.0040 0.033 F 0.195 1.28 1.45 0.011 0.0010 0.040 0.0039 G 0.152 1.18 1.59 0.011 0.0011 0.027 0.0042 0.015 H 0.362 1.89 1.24 0.014 0.0012 0.041 0.0039 I 0.163 1.52 1.78 0.009 0.0012 0.033 0.0041 J 0.155 1.21 1.63 0.011 0.0014 0.038 0.0042 0.025 K 0.158 1.22 1.66 0.013 0.0009 0.029 0.0046 L 0.134 1.02 0.81 0.011 0.0015 0.029 0.0043 M 0.183 1.35 1.72 0.009 0.0014 0.033 0.0042 N 0.202 1.23 1.64 0.013 0.0011 0.033 0.0040 O 0.154 1.24 1.51 0.010 0.0012 0.041 0.0044 0.032 P 0.084 1.12 2.44 0.013 0.0011 0.036 0.0048 Q 0.163 1.19 1.62 0.012 0.0011 0.036 0.0040 0.032 R 0.162 1.13 1.47 0.012 0.0008 0.039 0.0039 0.100 Chemical composition (unit: mass %, remainder: Fe and impurities) A_(C3) Ms Steel Cu Ni B Ca Mg REM Zr Bi (° C.) (° C.) A 858 429 B 0.0012 812 419 C 859 428 D 0.0009 847 418 E 0.0035 848 411 F 0.0010 858 421 G 0.0009 855 436 H 861 348 I 0.0020 862 424 J 858 434 K 855 431 L 876 471 M 0.100 0.100 0.0010 848 415 N 847 411 O 0.0010 865 438 P 852 441 Q 859 431 R 0.0010 862 435

A thus obtained hot-rolled steel sheet had a complex structure of ferrite and martensite, or of ferrite and bainite.

Meanwhile, the hot rolling conditions for test materials No. 6 and No. 22 are different from the above conditions. Test material No. 6 simulated a step for winding a hot-rolled sheet at room temperature by retaining the sheet in a range of from 740° C. to 660° C. for 2 sec, and cooling the same by water spraying to room temperature. Test material No. 22 simulated a step for winding a hot-rolled sheet at 670° C., by cooling the sheet by water spraying to 670° C., and then cooling the same slowly at 20° C./hour to room temperature.

A part of a hot-rolled steel sheet obtained as above was freed from a scale by pickling, then subjected to cold rolling to a sheet thickness of 1.6 mm, heated at from 780° C. to 900° C., and annealed under condition of cooling at an average cooling rate of 30° C./sec. However, test material No. 27 was heated at 920° C., and annealed under condition of cooling at an average cooling rate of 30° C./sec.

Each of the area rates of ferrite, martensite, and bainite in a steel sheet to be subjected to hot pressing was measured applying an EBSP (Electron Back Scatter Pattern) method. Specifically, cross-sections both in the rolling direction and in the direction vertical to the rolling direction were sliced out from a steel sheet to be subjected to hot pressing. The sliced out cross-sections were subjected to polishing and nital etching. Next, using a scanning electron microscope (SEM) equipped with an EBSP detector (trade name QUANTA 200, produced by FEI Company), an IQ image (image quality map: magnification 2000×) of EBSP was obtained for each sliced out cross-section by an EBSP analysis. Then, the respective area rates of ferrite, martensite, and bainite were determined as an average value of area rates measured respectively based on respective IQ images of EBSP for both cross-sections in the rolling direction and in the direction vertical to the rolling direction. In this regard, for an EBSP analysis the following conditions were set: acceleration voltage=25 kV, working distance=15 mm, and scan step=0.2 μm.

Further, an aspect ratio of ferrite in a steel sheet to be subjected to hot pressing was measured as follows. Specifically, cross-sections both in the rolling direction and in the direction vertical to the rolling direction were sliced out from a steel sheet to be subjected to hot pressing. The sliced out cross-sections were subjected to polishing and nital etching. Next, using a scanning electron microscope (SEM) equipped with an EBSP detector (trade name QUANTA 200, produced by FEI Company), an IQ image (image quality map: magnification 2000×) of EBSP was obtained for each sliced out cross-section by an EBSP analysis. Then, the aspect ratio of ferrite was determined as an average value of aspect ratios of each 50 ferrite crystal grains measured based on each IQ image of EBSP for cross-sections both in the rolling direction and in the direction vertical to the rolling direction. In this regard, for an EBSP analysis the following conditions were set: acceleration voltage=25 kV, working distance=15 mm, and scan step=0.2 μm.

The steel structures of steel sheets to be subjected to hot pressing are shown in Table 2.

TABLE 2 Test material Kind of Aspect ratio Area rate of Area rate of Area rate of No. Steel steel sheet of ferrite ferrite (%) martensite (%) bainite (%) *1 *2 1 A Hot-rolled 1.4 36 64 0 64 100 steel sheet 2 A Hot-rolled 1.3 36 64 0 64 100 steel sheet 3 A Coated steel 1.3 28 5 65 70  98 sheet 4 B Hot-rolled 1.5 35 0 60 60  95 steel sheet 5 C Hot-rolled 1.4 29 0 66 66  95 steel sheet 6 C Hot-rolled 1.6 32 0 64 64  96 steel sheet 7 D Hot-rolled 1.3  0 100 0 100  100 steel sheet 8 E Hot-rolled 1.4  0 100 0 100  100 steel sheet 9 F Hot-rolled 1.2 32 68 0 68 100 steel sheet 10 F Hot-rolled 1.2 32 68 0 68 100 steel sheet 11 G Hot-rolled 1.4 22 78 0 78 100 steel sheet 12 H Hot-rolled 1.3  0 5 78 83  83 steel sheet 13 I Hot-rolled 1.6 37 0 56 56  93 steel sheet 14 I Hot-rolled 2.1 40 60 0 60 100 steel sheet 15 I Cold-rolled 1.3 44 56 0 56 100 steel sheet 16 I Cold-rolled 1.4 44 56 0 56 100 steel sheet 17 J Hot-rolled 1.3 35 0 60 60  95 steel sheet 18 J Cold-rolled 1.6 43 57 0 57 100 steel sheet 19 K Hot-rolled 1.4 34 66 0 66 100 steel sheet 20 K Hot-rolled 1.3 34 66 0 66 100 steel sheet 21 K Cold-rolled 1.5 42 50 5 55  97 steel sheet 22 K Coated steel 1.7 36 6 52 58  94 sheet 23 L Hot-rolled 1.4 54 37 3 40  94 steel sheet 24 M Hot-rolled 1.3 33 67 0 67 100 steel sheet 25 M Hot-rolled 1.3 33 67 0 67 100 steel sheet 26 N Hot-rolled 1.2 45 0 32 32  77 steel sheet 27 N Hot-rolled 1.4 41 59 0 59 100 steel sheet 28 N Hot-rolled 1.6 41 59 0 59 100 steel sheet 29 O Hot-rolled 1.4 39 0 55 55  94 steel sheet 30 P Hot-rolled 1.5 57 43 0 43 100 steel sheet 31 Q Hot-rolled 1.3 37 63 0 63 100 steel sheet 32 Q Cold-rolled 1.9 21 79 0 79 100 steel sheet 33 R Hot-rolled 1.4 28 0 68 68  96 steel sheet 34 R Cold-rolled 1.4  9 91 0 91 100 steel sheet *1: The total area rate of martensite and bainite (%) *2: The total area rate of ferrite, martensite, and bainite (%)

The obtained steel sheets were heated in a gas furnace at an air fuel ratio of 0.85 and under the conditions set forth in Table 3. Then the heated steel sheets were taken out of the heating furnace, and after an air cooling time until hot pressing (time period from extraction of a sheet out of the furnace to placement of the same into a mold, namely time period in which a steel sheet is exposed to air cooling between the completion of heating and the initiation of hot forming) regulated to a time as set forth in Table 3, subjected to hot pressing using a flat plate steel-made mold. Then, after hot pressing, the steel sheets were cooled at an average cooling rate set forth in Table 3 down to 150° C., which was not higher than an M_(S) point, while keeping the steel sheets in contact with the mold, and thereafter taken out from the mold and left standing to allow cooling, thereby completing various test steel sheets (such a test steel sheet is hereinafter referred to as “hot-pressed steel sheet”).

Cooling was performed 1) after cooling the periphery of a mold with cooling water, 2) after cooling in a mold, which had been at normal temperature, or 3) after cooling in a heated mold, by cooling the periphery of a mold with cooling water. An average cooling rate down to 150° C. was determined by attaching a thermocouple to an edge of a steel sheet to be subjected to hot pressing, and reading the temperature. In this regard, a heating time means a time period from a time point when a steel sheet reaches 720° C. after placement of the same in a furnace, until the same is taken out from the furnace. Meanwhile, in Examples 6, 18 and 25, various test steel sheets were prepared by conducting gas cooling at a predetermined cooling rate after air cooling for a predetermined time period, for simulating a hot pressing condition, under which a cooling rate is changed using a mold with grooves,

The area rates of ferrite, tempered martensite, tempered bainite, and martensite of a hot pressed steel sheet were measured identically with the respective area rates of ferrite, martensite, and bainite of a steel sheet to be subjected to hot pressing, applying an EBSP (Electron Back Scatter Pattern) method. The results are shown in Table 4.

The aspect ratio of ferrite of a hot pressed steel sheet was measured identically with the aspect ratio of ferrite of a steel sheet to be subjected to hot pressing.

The mechanical properties of a hot pressed steel sheet were examined as follows. The results are also shown in Table 4.

JIS No. 5 test piece for tensile test was sampled from each steel sheet in the direction normal to the rolling direction, and a tensile test was carried out to measure TS (tensile strength) and El (total elongation).

Further, a rectangular sample was cut from each steel sheet allowing a bending ridge line to be directed normal to the rolling direction, and a surface thereof was machined to prepare a bending test piece with a thickness of 1 mm, a width of 30 mm, and a length of 60 mm. The test piece was subjected to a V-bend test with a tip angle of 90°, and tip radii of 5 mm, 4 mm, and 3 mm for evaluating the bendability. In this regard, the machined surface constituted an inner surface of a bend. The surface of a bend after the test was examined visually, and rated according to the following rating criteria.

—Rating Criteria of Bendability—

A: After a V-bend test with a tip radius of 4 mm, a crack is not recognized. B: After a V-bend test with a tip radius of 4 mm, a microcrack or necking is recognized. C: After a V-bend test with a tip radius of 4 mm, a crack is recognized. D: After a V-bend test with a tip radius of 5 mm, a crack is recognized.

Steel sheets produced in the present Example were not hot-pressed with a mold, however experienced the same heat history as a hot pressed steel sheet component. Therefore, the mechanical properties of a steel sheet were substantially the same as those of a hot pressed steel sheet component having the same heat history.

An underlined value in Table 1 to Table 4 means that a content, a condition, or a mechanical property expressed by the value is outside the scope of the invention.

TABLE 3 Average cooling Heating rate rate to from room Air temperature Test temperature Heating Heating cooling range of Ms material to 720° C. temperature time time point or less No. (° C./sec) (° C.) (min) (sec) (° C./sec) 1 12 800 5 10 70 2 12 900 4 15 70 3 12 800 5 10 70 4 12 775 5 10 70 5 12 800 5 10 70 6 12 800 5 10 15 7 12 800 5 10 70 8 12 790 5 10 70 9 12 800 5 10 70 10 12 800 5 10 400  11 12 800 5 10 70 12 12 790 5 10 70 13 12 800 5 10 70 14 12 840 5 10 70 15 12 800 5 10 70 16 12 820 5 25 70 17 12 800 5 10 70 18 12 800 5 10  8 19 12 800 5 10 70 20 12 680 5 10 70 21 12 800 5 10 70 22 12 800 5 10 70 23 12 800 5 10 70 24 12 800 5 10 70 25 12 800 5 10  5 26 12 800 5 10 70 27 12 775 5  5 70 28 12 775 5 17 70 29 12 800 5 10 70 30 12 800 5 10 70 31 12 800 5 10 70 32 12 770 8  1 70 33 12 800 5 10 70 34 12 740 5 10 70

TABLE 4 Steel structure Test Steel structure and Area rate (%) Ferrite Mechanical properties material Tempered Tempered Retained aspect TS EI No. Ferrite martensite bainite Martensite austenite *3 *4 ratio (MPa) (%) Bendability 1 25 39 0 36 0 39 100 1.4 1174 15 A Inventive Example 2 18 0 0 82 0  0 100 1.3 1275 10 D Comparative Example 3 15 2 50 33 0 52 100 1.3 1136 14 A Inventive Example 4 15 0 47 37 0 47  99 1.4 1102 11 A Comparative Example 5 22 0 46 32 0 46 100 1.4 1089 17 A Inventive Example 6 23 0 40 33 4 40  96 1.5 1027 16 B Inventive Example 7  0 58 0 42 0 58 100 1.3 1245 9 D Comparative Example 8  0 62 0 38 0 62 100 1.4 1305 11 D Comparative Example 9 23 44 0 33 0 44 100 1.2 1194 16 A Inventive Example 10 22 45 0 33 0 45 100 1.2 1243 14 B Inventive Example 11 15 49 0 36 0 49 100 1.3 1142 17 A Inventive Example 12  0 0 55 45 0 55 100 1.2 1486 7 D Comparative Example 13 26 0 38 36 0 38 100 1.5 1186 15 A Inventive Example 14  4 28 0 68 0 28 100 2.1 1198 8 D Comparative Example 15 32 33 0 35 0 33 100 1.3 1203 14 A Inventive Example 16 43 21 0 23 2 21  87 1.3 968 14 A Comparative Example 17 24 0 40 36 0 40 100 1.3 1256 14 A Inventive Example 18 31 31 0 31 7 37  93 1.5 1043 18 D Comparative Example 19 22 43 0 35 0 43 100 1.4 1178 16 A Inventive Example 20 34 66 0  0 0 66 100 1.3 848 16 A Comparative Example 21 33 28 3 36 0 31 100 1.4 1146 15 A Inventive Example 22 28 0 39 33 0 39 100 1.5 1137 14 A Inventive Example 23 52 18 0 23 0 18  93 1.4 925 15 D Comparative Example 24 26 39 0 35 0 39 100 1.2 1203 14 A Inventive Example 25 30 37 0  4 3 37  71 1.3 878 24 A Comparative Example 26 38 0 18 44 0 18 100 1.2 1278 13 D Comparative Example 27 36 30 0 34 0 30 100 1.3 1175 13 C Inventive Example 28 38 31 0 31 0 31 100 1.3 1136 17 C Inventive Example 29 25 0 43 32 0 43 100 1.3 1246 15 A Inventive Example 30 35 29 0 36 0 29 100 1.4 946 16 A Comparative Example 31 24 42 0 34 0 42 100 1.3 1143 15 A Inventive Example 32 17 51 0 26 6 51 100 1.8 1126 14 D Comparative Example 33 18 0 45 37 0 45 100 1.3 1189 14 A Inventive Example 34  7 85 0  8 0 85 100 1.3 949 10 A Comparative Example *3: The total area rate of tempered martensite and tempered bainite (%) *4: The total area rate of ferrite, tempered martensite, tempered bainite, and martensite (%)

The test materials No. 1, 3, 5, 6, 9, 10, 11, 13, 15, 17, 19, 21, 22, 24, 27, 28, 29, 31, and 33 as Inventive Examples in Table 4 are steel sheet components of Inventive Examples, namely hot pressed steel sheet components, satisfying all the requirements according to the invention. Any of the hot pressed steel sheet components of Inventive Examples as hot-formed has a tensile strength as high as 980 MPa or more, and superior in ductility as well as bendability.

Meanwhile, with respect to the test material No. 2, since the heating temperature of the steel sheet exceeded the upper limit of the range defined according to the invention, a desired structure could not be obtained and the ductility and the bendability were inferior.

With respect to the test material No. 4, since the Si content was below the lower limit of the range defined according to the invention, the ductility was inferior.

With respect to the test material No. 7, since the steel sheet to be subjected to hot pressing and the hot pressed steel sheet component did not have the structure defined according to the invention, the ductility and the bendability were inferior.

With respect to the test material No. 8, a desired structure was not obtained for the steel sheet to be subjected to hot pressing and the hot pressed steel sheet component, and the ductility and the bendability were inferior.

With respect to the test material No. 12, since the C content exceeded the upper limit of the range defined according to the invention, and the steel sheet to be subjected to hot pressing and the hot pressed steel sheet component did not have the structure defined according to the invention, the ductility and the bendability were inferior.

With respect to the test material No. 14, a desired structure was not obtained for the steel sheet to be subjected to hot pressing and the hot pressed steel sheet component, and the ductility and the bendability were inferior.

With respect to the test materials No. 16, 20, and 25, since the air cooling time, the heating temperature, and the average cooling rate fell outside the respective ranges defined according to the invention, a desired structure was not obtained for the hot pressed steel sheet component, and a targeted tensile strength was not obtained.

With respect to the test material No. 18, since the average cooling rate fell outside the range defined according to the invention, a desired structure was not obtained for the hot pressed steel sheet component, and the bendability was inferior.

With respect to the test material No. 23, since the Mn content was below the lower limit of the range defined according to the invention, and the steel sheet to be subjected to hot pressing and the hot pressed steel sheet component did not have the structure defined according to the invention, a targeted tensile strength was not obtained and the bendability was inferior.

With respect to the test material No. 26, since the steel sheet to be subjected to hot pressing and the hot pressed steel sheet component did not have the structure defined according to the invention, the bendability was inferior.

With respect to the test material No. 30, since the C content was below the lower limit of the range defined according to the invention, a targeted tensile strength was not obtained.

With respect to the test material No. 32, since the air cooling time fell outside the range defined according to the invention, a desired structure was not obtained for the hot pressed steel sheet component, and the bendability was inferior.

Further with respect to the test material No. 34, since the steel sheet to be subjected to hot pressing and the hot pressed steel sheet component did not have the structure defined according to the invention, the tensile strength was low, and the ductility was inferior.

The entire disclosures of Japanese Patent Application No. 2013-247814 are hereby incorporated by reference.

All the literature, patent application, and technical standards cited herein are also herein incorporated to the same extent as provided for specifically and severally with respect to an individual literature, patent application, and technical standard to the effect that the same should be so incorporated by reference. 

1. A steel component after hot pressing, comprising: a chemical composition comprising, in terms of mass %, C at from 0.100% to 0.340%, Si at from 0.50% to 2.00%, Mn at from 1.00% to 3.00%, P at 0.050% or less, S at 0.0100% or less, sol. Al at from 0.001% to 1.000%, and N at 0.0100% or less, with a remainder consisting of Fe and impurities; and a steel structure comprising ferrite, at least one of tempered martensite or tempered bainite, and martensite, wherein an area rate of ferrite is from 5% to 50%, a total area rate of tempered martensite and tempered bainite is from 20% to 70%, an area rate of martensite is from 25% to 75%, a total area rate of ferrite, tempered martensite, tempered bainite and martensite is 90% or more, and an area rate of retained austenite is from 0% to 5%.
 2. The steel component after hot pressing according to claim 1, wherein the chemical composition comprises at least one selected from the group consisting of Ti at 0.200% or less, Nb at 0.200% or less, V at 0.200% or less, Cr at 1.000% or less, Mo at 1.000% or less, Cu at 1.000% or less, Ni at 1.000% or less, B at 0.0025% or less, Ca at 0.0100% or less, Mg at 0.0100% or less, REM at 0.0100% or less, Zr at 0.0100% or less, and Bi at 0.0100% or less, in terms of mass %. 3-5. (canceled)
 6. A steel sheet for hot pressing, comprising: a chemical composition comprising, in terms of mass %, C at from 0.100% to 0.340%, Si at from 0.50% to 2.00%, Mn at from 1.00% to 3.00%, P at 0.050% or less, S at 0.0100% or less, sol. Al at from 0.001% to 1.000%, and N at 0.0100% or less, with a remainder consisting of Fe and impurities; and a steel structure comprising ferrite with an aspect ratio of 2.0 or less, and at least one of martensite or bainite, wherein an area rate of ferrite is from 5% to 50%, a total area rate of martensite and bainite is from 45% to 90%, and a total area rate of ferrite, martensite, and bainite is 90% or more.
 7. The steel sheet for hot pressing according to claim 6, wherein the chemical composition comprises at least one selected from the group consisting of Ti at 0.200% or less, Nb at 0.200% or less, V at 0.200% or less, Cr at 1.000% or less, Mo at 1.000% or less, Cu at 1.000% or less, Ni at 1.000% or less, B at 0.0025% or less, Ca at 0.0100% or less, Mg at 0.0100% or less, REM at 0.0100% or less, Zr at 0.0100% or less, in terms of mass %, and Bi at 0.0100% or less, in terms of mass %. 8-10. (canceled)
 11. A method for producing a steel component after hot pressing, the method comprising: heating the steel for hot pressing according to claim 6 to a temperature range of 720° C. or higher but lower than an Ac₃ point; performing hot pressing within a time period of from 3 sec to 20 sec, during which the steel sheet is exposed to air cooling from the end of the heating until the initiation of the hot pressing; and cooling to a temperature range not above an M_(S) point at an average cooling rate of from 10° C./sec to 500° C./sec. 